by Suzanne Harvey

The walrus penis bone, also known as an os penis or baculum, is one of the most popular objects at the Grant Museum. The human penis is haemodynamic, meaning an erection is achieved by blood pressure alone. In animals with an os penis, blood pressure still plays an important role, but the pressure functions to push a bone structure into the penis in order to achieve an erection. This has many benefits over an erection sustained by blood pressure alone, not least in keeping the glans open for sperm to pass through.

While the importance of shaft size and sperm competition has been discussed in my previous blog post, even the largest penis will offer no evolutionary advantage if sperm cannot escape: these much desired qualities will never be passed to offspring. This is not the only benefit. The os penis increases the potential duration of intercourse and also the frequency with which intercourse can take place. For example, a lioness can copulate 100 times per day, sometimes with only four minute intervals, but has only a 38% conception rate1 – males need to keep up if they’re to achieve the best chance of paternity. It comes as a surprise to many people that the os penis exists at all, but in fact humans, woolley monkeys and spider monkeys are the only primates to lack this handy piece of anatomy.

With such benefits to structure, strength, endurance and recovery time, the next question must be: where is man’s penis bone? This is where the story becomes more intriguing…

Was it sacrificed for the sake of woman?

While the lack of a human penis bone may have interested evolutionary scientists for many years, it has recently attracted attention from professors of Theology as well. The debate was reignited in 2001 with the publication of a letter entitled Congenital human baculum deficiency: the generative bone of Genesis 2:21-23. Without wishing to delve into a religion vs. science debate, perhaps the most surprising aspect of this article is that it was published in the American Journal of Medical Genetics, with its central theory that God took Adam’s os penis to create Eve in the Garden of Eden, rather than his rib:

Ribs lack any intrinsic generative capacity. We think it is far more probable that it was Adam’s baculum that was removed in order to make Eve. That would explain why human males, of all the primates and most other mammals, did not have one.2

The authors then go on to cite possible mistranslations of the Hebrew word ‘tzela’, which can be translated as ‘rib’ or more generally as a supporting structure such as a baculum. More recently however, a study using the dissection and microscopic analysis of human, dog and rat penises revealed something rather unexpected…

Perhaps it was there all along!

With the comprehensive list of benefits discussed above, the root of this problem seems to be in discovering how on earth humans manage to function without an os penis. As a rule, larger penises in humans can give an evolutionary advantage, but without the toughness of structure provided by a bone, how do they avoid buckling under the pressure? Well, through both microscopic analyses and collagen tissue staining, the elusive evidence has finally been discovered. In the core of the human penis, there is a tunica (or lining) with many elastic fibres acting to keep the penis rigid in much the same way as an os penis. This has been referred to as the distal ligament, and is a structure so robust that even after some venous removal, erection can still be achieved without the need for a fully formed bone structure.4 In somewhat less good news for men, the authors of this study further hypothesise that if damaged, the distal ligament may take as long as a broken bone to heal.

So we are left with the final puzzle of why man lost his penis bone but retained a structure that serves a similar purpose. For the answer to this, we may need to look to women. Sexual selection works on the premise of ‘honest signals’. Put simply, females need to select high quality males to ensure the best genes for their offspring, and a penis that works predominantly through haemodynamics takes a lot of energy to produce. Therefore this is an honest signal of the most healthy males in the same way that the healthiest stags grow the largest antlers. In the end, for all the dissection, microscopic analyses and experiments with cadavers, Adam may have given up his penis bone for Eve after all.

Suzanne Harvey is a PhD student in Biological Anthropology, working on social interactions and communication in wild olive baboons. She is also ateaching assistant on the UCL Arts and Sciences BASc, a new interdisciplinary degree, and can be found on twitter @suzemonkey.

Fascinating! Particularly the insight into the meaning of ‘tsela’. I spoke about this theory at Science Showoff this week, raising the point that spider monkeys and wooly monkeys are the only other primates that don’t have a baculum. I wonder how the Genesis explanation can accommodate the fact humans are not unique? Thanks for the response!

[…] selection for appearance. The human penis is not only larger, but unlike most mammals, lacks a penile bone, or baculum. The mystery was solved when scientists arrived at the conclusions that achieving an […]

We lost our penile bone not because women pick their men based on penis size, but because of male-male competition in the form of a war of the penis. The war had begun in our ape ancestor. The chimp lives in groups with multiple males, who cooperate for group defense. Unlike gorillas, male chimps cannot exclude other males from mating with the females. Simultaneously, female chimps want to have sex with multiple males to reduce the risk of infanticide. If each male thinks that he is the father, he will be less likely to try to kill the baby. Males in turn need each other’s good will to hunt monkeys cooperatively and to defend their territory as a group. Therefore they cannot beat each other up over the ladies.

That means males cannot compete through exclusion, but they can compete in other ways. The way to compete is to have a longer penis so sperm can be placed closer to the uterus. Accordingly, male chimps have much longer penises than male gorillas, and they also have large testicles to store more sperms. Think about it, why would female chimps prefer long penises, but female gorillas don’t? But since chimps retain the penile bone, there is a limit to penis size, so they compensate by having testicles that are larger even then those of humans. Humans, by losing the penile bone, have evolved the largest penis (both in absolute size and in proportion to the body) in order to compete, so we don’t need to store as much sperms as chimps. One may ask why do humans have to compete like male chimps, since modern humans are largely monogamous? The answer is that we first evolved in the open African savanna, in which our ancestors must band together to defend themselves against predators. Therefore our ancestors too cannot afford to beat each other up over the ladies. Instead they engage in a cold war of sorts by evolving ways to make sure their sperms have the best chance of being the one that impregnates the females, who are promiscuous. Ironically, since humans have invented agriculture, we no longer need to travel around in large groups. Men can now keep their women from having sex with other men by inventing and enforcing new laws that prohibit adultery, because men can now settle down with a small plot of land and be safe from predators, while growing enough food to feed a family. Penis size is no longer subject to as much natural selection as it used to be, because, men with a small penis can still leave his share of descendants if he has the means to raise a family.

[…] selection for appearance. The human penis is not only larger, but unlike most mammals, lacks a penile bone, or baculum. The mystery was solved when scientists arrived at the conclusions that achieving an […]